Overhead projectors of this kind are known. They are distinguished by their very flat base plate from another design of overhead projectors in which the light source is arranged in a large square casing underneath the transparent writing plate and the Fresnel lens. By virtue of the small base plate, the overhead projector can be placed on a table and operated by a speaker seated at the table which is not the case with the other type of overhead projectors with the lamp arranged underneath the writing plate and Fresnel lens.
In the first mentioned type of overhead projector in which the lamp is arranged above the writing plate and a reflective Fresnel lens, it is customary for the objective lens to be arranged with a vertical axis and for a deflection mirror to be arranged above the objective. Another type of overhead projector has the century old arrangement that the optical axis of the objective is almost horixontal and the deflection mirror is arranged on the side of the objective which is associated with the path of rays of the Fresnel lens.
In particular, when the overhead projector is used to project a picture of the original on a projection surface which has a height between 1 mm and 3 mm, the projection head with the support arm of the overhead projector appears disturbingly in the field of view of the spectators. This disturbance has heretofore been accepted because one would not allow too great a picture distortion and light strength difference in the projected picture. With a projection head arranged lower, it is namely necessary to tilt the deflection mirror so that the deflection of the optical path of rays does not lie in the vicinity of 90.degree., but greatly exceeds the optimal angle of 90.degree. . However, the arrangement of the projection head high above the writing surface is objectionable not only because of being in the field of view of spectators, but also by reason of requiring a very long support stem which because of its great length is inconvenient for transport and by reason of its great length must be very stiff so that the projection head during the projection does not get into oscillation or otherwise wobble.